This invention relates to a processor architecture, and in particular to an architecture which can be used in a wide range of devices, such as communications devices operating under different standards.
In the field of digital communications, there has been a trend to move as many functions as possible from the analogue domain into the digital domain. This has been driven by the benefits of increased reliability, ease of manufacture and better performance achievable from digital circuits, as well as the ever decreasing cost of CMOS integrated circuits. Today, the Analogue-Digital and Digital-Analogue Converters (ADC's and DAC's) have been pushed almost as near to the antenna as possible, with digital processing now accounting for parts of the Intermediate Frequency (IF) processing as well as baseband processing.
At the same time, there has been a vast improvement in the capability of microprocessors, and much of the processing for many narrowband communications systems is now performed in software, an example being the prevalence of software modems in PC's and consumer electronics equipment, partly because a general purpose processor with sufficient processing power is already present in the system. In the field of wireless communications there is extensive research in the field of software radio, the physical layers of broadband communications systems require vast amounts of processing power, and the ability to implement a true software radio for third generation (3G) mobile communications, for example, is beyond the capability of today's DSP processors, even when they are dedicated to the task.
Despite this, there has never been a time when there has been more need for software radio. When second generation (2G) mobile phones were introduced, their operation was limited to a particular country or region. Also, the major market was business users and a premium could be commanded for handsets. Today, despite diverse 2G standards in the USA and different frequency bands, regional and international roaming is available and handset manufacturers are selling dual and triple band phones which are manufactured in their tens of millions. After years of attempts to make an international standard for 3G mobile, the situation has now arisen where there are three different air interfaces, with the one due to replace GSM (UMTS) having both Frequency and Time Division Duplex (FDD and TDD) options. Additionally, particularly in the USA, 3G systems must be capable of supporting a number of legacy 2G systems.
Although a number of DSP processors are currently being developed that may be able to address the computational requirements of a 3G air interface, none of these show promise of being able to meet the requirements of a handset without the use of a number of hardware peripherals. The reasons for this are power and cost and size. All three are interrelated and controlled by the following factors:
1. The need for memory. Classical processor architectures require memory to store both the program and data which is being processed. Even in parallel Very Long Instruction Word (VLIW) or Single Instruction Multiple Data (SIMD) architectures, the entire processor is devoted to one task at a time (eg: a filter, FFT or Viterbi decoding), with memory required to hold intermediate results between the tasks. In addition, fast local instruction and data caches are required. Altogether, this increases the size and cost of the solution, as well as dissipating power. In hard-wired architectures, data is usually transferred directly from one functional block to another, with each block performing DSP functions on the data as it passes through, thus minimising the amount of memory required.
2. Data bandwidth. In hard-wired solutions, all data is held locally, if necessary in small local RAM's within functional blocks. Some transceivers may contains several dozen small RAM's, and although the data bandwidth required by each RAM may be relatively small, the overall data bandwidth can be vast. When the same functions are implemented in software running on a processor, the same global memories are used for all data and the required data bandwidth is enormous. Solutions to this problem usually involve the introduction of local memories in a multi-processor array, but the duplication of data on different processors and the task of transferring data between processors via Direct Memory Access (DMA) mean that the power dissipation is, if anything, increased, as is silicon area and consequently cost.
3. The need for raw processing power. In today's DSP processors, improvements in processing throughput are achieved by a combination of smaller manufacturing process geometries, pipelining and the addition of more execution units (e.g. arithmetic logic units and multiplier-accumulators). Improvements in manufacturing processes are open to all solutions, and so are not a particular advantage for conventional DSP processors. The other two methods both come with considerable overheads in increased area and power, not merely because of the extra hardware which provides the performance improvement, but because of the consequential increases in control complexity.